This protocol is designed to test the hypothesis that continuing a program of regular, sustained, recreational exercise throughout pregnancy does not produce evidence of significant fetal hypoxia at term. It is based on three earlier observations. First, that splanchnic blood flow falls during exercise. Second, that sustained exercise is associated with a rise in fetal heart rate which probably represents a sympathetic response to a minimal fall in fetal pO2. Third, that amniotic fluid erythropoietin levels are elevated in clinical states associated with chronic hypoxia. In order to test this hypothesis, samples of amniotic fluid and umbilical vein blood will be obtained during labor and delivery from two groups of women: women who continued a regular regimen of exercise and a matched group of women who did not.